Compression Ratio and Minimum Cylinder Volume (CR/MCV)

I. General Description

The CR/MCV subsystem is designed to precisely measure the compression ratio and minimum cylinder volume of spark-ignition and diesel internal combustion engines. The subsystem is designed to be used in conjunction with a standard Jodon Laboratory Microwave
Engine Timing System (METS®). The METS® equipment is required because of the need for accurate measurement of piston TDC and crankshaft position.

II. Measurement Method

The CR/MCV measurement method uses accurately measured cylinder pressure data, engine geometry information, and a polytropic thermodynamic model of the engine cylinder. A portion of the cold-motored compression or power stroke that is well modeled by
the polytropic equation is chosen for analysis.

Engine geometry information for the test engine is entered via the keyboard and stored in the computer memory.
Nominal design values are used for parameters such as cylinder bore diameter and connecting rod length. This information is used to compute the nominal change in volume between successive pressure data measurement positions.

Cylinder pressure data is averaged over at least one hundred engine cycles. This data is acquired during cold motoring of the engine at 600 to 1100 RPM. (Or, during combustion-driven operation, the test cylinder must be disabled temporarily). The interval between data points is approximately 0.2 crankshaft degrees.

After pressure data acquisition, a large number of computations are performed in order to arrive at a composite value for minimum cylinder volume (MCV) at piston top dead center. This composite MCV value represents the average effective MCV value for the analyzed region of the engine cycle. These computations also test the extent to which the data is well modeled by the polytropic equation, and poorly modeled data is rejected.

The computed MCV value is used with the nominal engine geometry information in order to compute compression ratio (CR). MCV and CR are then displayed on the terminal and printer, along with other relevant information.

III. Utilization Checklist
The items described below are required for proper utilization of Jodon’s system for measurement of compression ratio and minimum cylinder volume (CR/MCV):

  1. From a microwave engine timing system (METS®):
    1. A TTL-compatible once-per-revolution pulse corresponding to piston top dead center (TDC). This pulse is derived from microwave data collected in the range of 500 to 1100 RPM.
    2. Crankshaft rotational reference signals consisting of two quadrature 900 (approximate) pulse-per-revolution signals and a one-per-revolution zero index pulse. These signals are derived from an optical encoder, ring gear sensor, or similar means of torsionally-stiff crankshaft instrumentation.
  2. Capability for cold motoring over at least the range of 600 to 1200 RPM. If cold motoring capability is not available, then it is normally acceptable to temporarily disable one cylinder of a combustion-driven engine for CR/MCV measurement.
  3. Access hole for a cylinder pressure probe for the cylinder to be measured. Typically, an existing hole for the spark plug, glow plug, or fuel injector can be used. Also, on research engines, a pressure port is sometimes available.
  4. Intake manifold air pressure monitoring.
    The cylinder pressure probe (piezoelectric type) responds only to variations in cylinder pressure. Therefore, in order to reference this measurement to absolute pressure, intake manifold air pressure at intake/compression
    bottom dead center must be known. The CR/MCV system can accept this information in one of two ways:

    1. If the intake manifold has unrestricted access to atmospheric air, then barometric pressure can be entered via the keyboard.

    2. If the intake manifold has unrestricted access to atmospheric air, then an absolute pressure sensor (e.g., a piezoelectric type) must be used to measure pressure in the air stream at or near the intake manifold.
  5. Crankcase oil temperature and intake manifold air dew point monitoring. the CR/MCV measurement is slightly sensitive to variations in these parameters. The CR/MCV measurement repeatability can thus be improved either by maintaining these parameters at constant
    levels or by adjusting the CR/MCV measurement for changes in these parameters.

    1. Long term repeatability for raw CR/MCV data is approximately +/- 2% for spark ignition engines and +/- 5% for diesel engines.

    2. Long term repeatability for manually compensated CR/MCV data is approximately +/- 1% for spark ignition engines and +/- 2.5% for diesel engines. These figures are expected to improve with continuing CR/MCV system development.

  6. Since the CR/MCV measurement is based on cold-motored cylinder pressure data, the test engine must not exhibit more than a normal level of cylinder air leakage (i.e., blow-by, valve leakage, etc.).

  7. Prior to use as a measurement tool for a given engine model or type, the CR/MCV system should be calibrated using one or more engines of the same model or type for which minimum cylinder volume has already been measured by alternative means, such as liquid displacement.

  8. Prior to use as a measurement tool for a given engine model or type, the CR/MCV system requires keyboard entry of certain engine design information, such as firing order, cylinder bore diameter, stroke, crankshaft offset, wrist pin offset, and approximate minimum cylinder volume. This information can then be stored for future reference in nonvolatile memory as a CR/MCV engine code.